This invention relates to the so-called zinc hot dipping process, and more particularly, to a processs for producing a one-side zinc hot dipped steel sheet by the utilization of a water-glass type anti-plating agent. More specifically, this invention is concerned with the baking of an anti-plating coating capable of preventing entrainment of zinc to the anti-plating film and ensuring complete removal of the antiplating film.
The applicant has made a number of proposals for the manufacture of one-side zinc hot dipped steel sheets using anti-plating agents based on water-glass (alkali metal silicates), boric acid, magnesium oxide, alkali metal hydroxides, etc. and containing an additive in the form of a metal oxide such as TiO.sub.2 and Al.sub.2 O.sub.3. This one-side zinc hot dipping process using an anti-plating agent takes advantage of little adhesion between a film of such an anti-plating agent and molten zinc. The anti-plating agent is applied to a steel sheet and then dried and baked to form an anti-plating film before the steel sheet is immersed in a molten zinc bath. It has been found that the adhesion between the anti-plating film and molten zinc varies with the baking conditions since the surface nature of the anti-plating film largely depends on the baking conditions. Depending on the conditions under which the anti-plating coating is baked, there is a likelihood that zinc will be deposited or entrained on the surface of the anti-plating film. Such entrained zinc particles are carried with the continuously moving steel sheet and transferred to rolls and other transport members in the system, making dents in the steel sheet. After plating of the other surface of the steel sheet with zinc, the antiplating film is removed from the sheet by subjecting the sheet to bending by suitable bending means such as a roll bender, thereby causing the anti-plating film to peel off through the utilization of a difference in flexural strength between the steel sheet and the anti-plating film. Should a few number of zinc particles be entrained on the anti-plating film, dents or flaws would occur in the steel sheet during the peeling processs. In some cases, the anti-plating film cannot be completely removed from the steel sheet by means of relatively simple bending machinery.